Entacapone, an inhibitor of catechol-O-methyltransferase (COMT), is a nitro-catechol-structured compound with a molecular weight of 305.3. It is used in the treatment of Parkinson's disease as an adjunct to levodopa/carbidopa therapy. The chemical name of entacapone is (E)-2-cyano-3-(3,4-dihydroxy-5-nitrophenyl)-N,N-diethyl-2-propenamide. Its empirical formula is C14H15N3O5, and its structural formula is:

Carbidopa, an inhibitor of aromatic amino acid decarboxylation, is a white, crystalline compound, slightly soluble in water, with a molecular weight of 244.3. Chemically, it is (−)-L-(α-hydrazino-(α-methyl-β-(3,4-dihydroxybenzene)propanoic acid monohydrate. Its empirical formula is C10H14N2O4H2O, and its structural formula is:

Levodopa, an aromatic amino acid, is a white, crystalline compound, slightly soluble in water, with a molecular weight of 197.2. Chemically, it is (−)-L-α-amino-β-(3,4-dihydroxybenzene)propanoic acid. Its empirical formula is C9H11NO4, and its structural formula is:

U.S. Pat. No. 4,963,590 discloses a pharmaceutical composition comprising entacapone and pharmaceutically acceptable carrier.
U.S. Pat. No. 6,599,530 provides an oral compacted composition in the form of a tablet, which comprises entacapone, nitecapone, or pharmaceutically acceptable salt of entacapone or nitecapone, and croscarmellose sodium in an amount of at least 6% by weight of the composition.
U.S. Pat. Nos. 6,500,867 and 6,797,732 disclose oral solid tablet compositions comprising entacapone, levodopa and carbidopa, or pharmaceutically acceptable salts or hydrates thereof, and a pharmaceutically acceptable excipient. Both these patents disclose that when carbidopa, levodopa and entacapone are mixed together, it results in stability problems and desired therapeutic effect is not achieved. On the other hand, when a substantial portion of carbidopa is separated from levodopa and entacapone, the formulation exhibits better stability and desired therapeutic effect is also achieved.
US Application No. 20060222703 describes oral pharmaceutical compositions of entacapone, carbidopa and levodopa with microcrystalline cellulose and starch by simultaneous mixing of all the three actives. The composition is prepared by compaction granulation. The application describes the disadvantages associated with wet granulation technique which includes destabilization of composition and decreased dissolution of levodopa, carbidopa and entacapone due to use of water in wet granulation method.
The present invention addresses and overcomes these commonly encountered problems.